1. Field of the Invention
The present invention relates to ultrasonic transducers made from piezoelectric ceramic polymer composite materials, and, more particularly, to ultrasonic transducers made from a multi-frequency composite structure that broadens the transducer bandwidth, and to methods for making such transducers.
2. Background
In general, ultrasonic transducers are constructed by incorporating one or more piezoelectric vibrators which are electrically connected to pulsing-receiving system. Conventionally, the piezoelectric member is made up of a PZT ceramic, a single crystal, a piezo-polymer composite or piezoelectric polymer. The transducers are shaped in plate form (a single element transducer) or in bars (a slotted array transducer) and the parallel opposite major surfaces thereof (which extend perpendicularly to the propagation direction) have electrodes plated thereon to complete the construction. When the piezoelectric is subjected to mechanical vibration and electrically excited, acoustic waves are then transmitted to the propagation medium with a wavelength according to the thickness of the piezoelectric. Thus, the nominal frequency of an ultrasonic transducer is obtained by determining the dimension of piezoelectric in the direction of propagation. Based on these considerations, ultrasonic transducers exhibit a unique nominal frequency that corresponds to the thickness resonance mode and thus the bandwidth of such transducers is inherently limited or bounded. A common task facing transducer designers is the optimization of the efficiency of, or otherwise improving, the electromechanical coefficient of the transducer which determines the quality of the transducer device. The most common technique of producing piezo-ceramic based ultrasonic transducers involves the provision of a backwardly damping member or backing member and/or an impedance matching layer at the transducer front face. In the first case, the sensitivity of the transducer decreases proportionally to the increase in the backing impedance, and, therefore, according to the bandwidth provided, while an improvement in both sensitivity and bandwidth can be provided by the use of a matching layer.
In practice, ultrasonic transducers are based on a judicious compromise with respect to the ratio of gain-bandwidth, and thus commonly use a medium impedance backing associated with a single or a double matching layer to achieve satisfactory performance. The set of double matching layers is composed of a first layer attached to the front surface of the piezoelectric and having an acoustic impedance between that of piezoelectric and the second matching layer, a second layer attached to the external face of the first layer and having impedance lower than that of the propagation medium. In this way, a gradient of acoustic impedances is obtained between the piezoelectric and the propagation medium, and the impedance value of each component is calculated based on a polynomial function to minimize reflection at the various interfaces.
Although the optimization techniques described above will enable transducer to provide a fractional bandwidth up to 70-80%, because of the compromise that must be accepted, the transducer sensitivity may decrease dramatically (with a heavy backing) or the fabrication of transducer may be complicated (e.g., with more than two matching layers). During the past decade, such bandwidth (i.e., a bandwidth on the order of 70%) provides acceptable performance when using standard medical diagnostic equipment or systems equipped with low dynamic range image processors. However, with the introduction of harmonic imaging techniques and full digital imaging mainframes, modern systems can now accept, and even require, an extended bandwidth scan-head to take advantage of the potential of these new technologies.
To provide the market with improved transducer products, manufacturers have made a number of new developments. One of these concerns the use of high mechanical loss piezoelectric material such as a polymer or ceramic-polymer composite. The particular structure of these materials allow increased damping of the transducer so that the impulse response is enhanced. The gain in bandwidth is about 5 to 10% with a composite and more with piezoelectric polymer but in the latter case, this increase in bandwidth is associated with a dramatic decrease in sensitivity.
Another direction which this recent research has taken focuses on multi-layer transducer structures wherein the piezoelectric device is produced by superposition of a plurality of reversed polarity single layers. The objective is to reduce the electrical mismatch between the piezoelectric impedance and those of the cable so as to minimize reflections at interface. Ringing is therefore shorter and sensitivity is improved. Unfortunately, the construction of such devices is highly difficult and requires large quantity production in order to be cost effective.
Still other techniques for broadening transducer bandwidth concern the use of a ceramic of non-uniform thickness. These techniques involve the provision of piezoelectric devices shaped to provide gradient thickness along the elevation dimension thereof so as to afford frequency and bandwidth control of the elevation aperture size and position, as well as the elevation focal depth. Transducers employing these techniques are described, for example, in the following U.S. Pat. No. 3,833,825 to Haan; U.S. Pat. Nos. 3,470,394 and 3,939,467 both to Cook; U.S. Pat. No. 4,478,085 to Sasaki; U.S. Pat. No. 6,057,632 to Ustuner; U.S. Pat. No. 5,025,790 to Dias; and U.S. Pat. No. 5,743,855 to Hanafy.
Briefly considering these patents, in the Haan patent, a thickness-mode transducer is provided which comprises an active body having non-parallel major surfaces for transmitting or receiving energy. The major surfaces of transducer are planar so that the transducer device provides a continuous variation in the resonance frequency from one edge thereof to the other.
The transducers as described in the Cook patents are of a serrated or even double serrated construction and have major opposite surfaces formed at an angle (the ""467 patent). Further, the transducer front face may be of convex or concave shape.
The Sasaki patent describes transducers having an element thickness which increases from the central portion toward both edges in elevation direction. However, the variation in thickness described herein is only of two types: continuous and stepwise. The purpose of the thickness variation described in this patent is to control the acoustic radiating pattern of transducer, and neither the manufacturing method used nor the actual transducer construction are fully addressed.
Similarly, the Dias patent discloses a variable frequency transducer wherein the piezoelectric member has a gradient thickness between the center thereof and the outermost ends. Each portion has a particular thickness corresponding to a desired frequency. As a consequence, the transducer provides discrete frequencies and the frequency characteristics are not compatible with the smooth bandwidth shape required by imaging transducers.
In the transducers disclosed in the Ustuner patent, the spacing of elements increases from the first end to the second end so that the dimensions of the overall transducer array tend to be those of a trapezoidal, thereby inherently limiting the number of elements in the array.
In the Hanafy patent, a gradient transducer is produced by grinding a thicker ceramic plate to provide the desired curvature, using a numerically controlled machine. However, machining a curved surface, and especially a cylindrical surface with perfect alignment relative to the ceramic edges has been found to be a particularly delicate operation which requires superior precision with respect to the tooling used and the process employed. Thus, fabrication method described in the Hanafy patent is difficult to carry out in practice. Moreover, if the machined surface profile must be mounted on or another piece of equipment for polishing or grounding (as in the case of a high frequency transducer), the operation can be very time consuming because the necessary positioning of the piezoelectric member requires additional tooling and control of the interfitting of the surfaces involved. Further, the Hanafy patent largely relates to gradient thickness transducers which have been described in other patents and which do not address the problems associated with the prior art manufacturing processes and associated machining requirements.
In accordance with the invention, a multi-frequency transducer is provided which overcomes or reduces the various drawbacks and disadvantages encountered in the prior art, including that represented by the above discussed patents. More particularly, the present invention relates to ceramic-polymer composite transducers and to new manufacturing methods for making such transducers, these methods being applicable whatever the geometry and shape of the particular transducer involved.
In general, three techniques or approaches are provided in accordance with the invention to broaden transducer bandwidth. In a first approach or aspect of the invention, grinding of piezoelectric composite member is provided to produce a graded thickness. Preferably, the resonance frequency of the resultant transducer decreases from the central portion to the outermost portion of the transducer. However, it will be understood that the method of the invention is not limited to this embodiment, and the method can be used to provide any desired variation in the thickness of the composite member and any ratio between thinnest and thickest portions thereof, according to the bandwidth required.
In accordance with a further aspect of the invention, a composite member is provided wherein the longitudinal velocity thereof varies from the center portion to the outermost portion of the composite of the composite member so that the resonance frequency thereof, which is a function of the longitudinal velocity, will vary proportionally.
A third aspect of the invention relates to a combination of the first two aspects mentioned above wherein a judicious compromise is arrived at to optimize the performance of the transducer as well as the manufacturing process used to make the transducer.
According to the first aspect of the invention, there is provided a manufacturing method for making a composite ultrasonic transducer so that the composite member has a curved or bent shape, this method comprising: forming (or thermo-forming) a composite member on a non-planar tooling device, firmly maintaining the composite member on the tooling device, grinding the upper surface of composite until an upper planar area is produced, metallizing the major surfaces of the composite member and completing construction of the transducer by affixing backing and matching layers as well as suitable connections.
The planar area obtained by grinding need necessarily not cover the entire surface of composite member at which grinding is carried out and the composite member may be formed in a concave or convex shape without changing the basic manufacturing process.
The forming or deformation of the composite member may also be performed on a surface having a three-dimensional curvature so a thickness variation is effected in both azimuthal and elevational planes.
Moreover, the curved surface is not necessarily of a spherical shape. In this regard, the shape of the surface may have a progressive curvature, an ellipsoid shape or a combination of curvature and sloping planes or the like.
As the resonance frequency of transducer changes shape, the matching layer or layers must be determined accordingly, so as to ensure that the thickness of matching layer or layers varies inversely with the frequency of transducer. The manufacturing process used in obtaining such a matching layer or layers is preferably similar to that used in making the composite member itself.
In a further preferred embodiment, the composite member is of regular thickness and the longitudinal sound velocity varies in the elevational plane, preferably from the center to the outermost end, but also from one end to the other end. In a preferred implementation, the composite member is ceramic ratio shifted, i.e., the longitudinal velocity is controlled by controlling the volume ratio of the ceramic material to the piezoelectric polymer material. In one advantageous embodiment, the ceramic ratio is higher at the center of transducer than the edges. Because the sound velocity in the ceramic material is typically twice that in polymer, a variation of the ratio of ceramic to the polymer will strongly affect the overall velocity in the composite member.
As indicated above, a third aspect of the invention involves a combination of the grinding technique or operation discussed hereinbefore with shifted velocity composite approach. The result is a smoothing of composite curvature in maintaining the enhancement of bandwidth previously mentioned. It should be noted that providing shifted behavior in a transducer presents difficulties and is more expensive than standard methods so that a judicious compromise should be made based on the geometrical specifications and requirements of the particular transducer being made.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.